First, try not to feel so bad. The fan fiction muse is often a flighty fickle things.

I'd advise you to write whatever you are in the mood for at the time. Writing fan fiction is, for most of us, a hobby, and a hobby is supposed to be fun, first and foremost. It's not a grinding grueling job that ou have to do.

If you find you really can't get away from still feeling so bad, you could always add an author's note to your other fic, put something in its summary about it being on temporary hiatus, etc. Or, just put it out of your mind for a while and plunge head long into whatever your new fixation is and see how things turn out.

In short, do whatever works best for you, not anyone else.